This invention relates generally to minimizing NO.sub.x production in gas turbine combustors and more particularly concerns reducing NO.sub.x emissions by mixing a small portion of combustion air into the fuel lines upstream of the premixer section of the combustor.
Hydrocarbons are widely used as fuels in gas turbine combustors. Because of its projected widespread availability, natural gas is an especially attractive fuel for stationary power generating systems. However, the emissions from the combustion of hydrocarbons lead to many environmental problems such as acid rain, ozone depletion and the "greenhouse" effect. Combustion by-products which pollute the atmosphere are required to be minimized as part of a growing concern for the quality of the environment. Oxides of nitrogen (NO.sub.x) are particularly undesirable by-products.
During combustion, NO.sub.x is formed in part by reactions which occur between atmospheric nitrogen and oxygen atoms. Because of the high activation energy of these reactions, NO.sub.x formation is not significant at temperatures below approximately 1800 K. to 1900 K. The requirement of high temperatures has led to water or steam injection for NO.sub.x control in conventional (non-premixed) combustors. In this approach, the injected water or steam absorbs heat, reduces the peak temperatures below the NO.sub.x -forming threshold, and so reduces NO.sub.x formation. However, this approach is expensive in terms of water or steam, can cause corrosion, and can increase carbon monoxide emission levels. Another common approach, injecting ammonia-based thermal deNO.sub.x into the exhaust stream, minimizes NO.sub.x, but this method is very expensive in terms of capital equipment and process ammonia.
Lean premixed combustion of gaseous hydrocarbons is an attractive approach because it offers relatively clean combustion without the need for post-combustion treatment of the exhaust. Typically, lean premixed combustion is accomplished by premixing fuel and air just upstream of flame stabilization (in a section of the combustor called the premixer section) to form a mixture on the lean side of stoichiometric. The effect of premixing is to reduce and hopefully minimize the temperature at which the mixture burns, thus reducing NO.sub.x production which is temperature sensitive. Generally, the leaner the mixture, the lower the combustion temperature will be.
The premixer sections of combustors used in industry typically fall short of completely mixing the gaseous fuel and air, thus resulting in higher levels of NO.sub.x. Efforts to improve the "mixedness" of fuel and air within the premixer by altering geometries and flow patterns have been attempted. However practical designs are constrained by such factors as allowable pressure drop limits and space limitations. Despite improvements in premixer designs, some incomplete mixing generally exists. Incomplete mixing leads to nonuniformities and fluctuations in fuel concentration levels during lean premixed combustion. Such nonuniformities and fluctuations cause increased NO.sub.x production because NO.sub.x production increases nonlinearly with the fuel concentration. For example, a fluctuation above the average fuel concentration will add more NO.sub.x than an equal fluctuation below the average fuel concentration will reduce. The net effect is that fluctuations produce more NO.sub.x than if combustion was performed at a constant fuel concentration. Similarly, nonuniform mean profiles contribute to greater NO.sub.x.